1. Field of the Invention
The present invention relates to a plasma display apparatus and driving method thereof, and more particularly, to a plasma display apparatus realizing gray levels and a driving method thereof.
2. Background of the Related Art
In a conventional plasma display panel, a barrier rib formed between a front panel and a rear panel forms one unit cell. Each cell is filled with a primary discharge gas, such as neon (Ne), helium (He) or a mixed gas of Ne and He, and an inert gas containing a small amount of xenon. If the inert gas is discharged with a high frequency voltage, it generates vacuum ultraviolet rays. The vacuum ultraviolet rays excite phosphors formed between the barrier ribs, thus implementing images. This plasma display panel can be manufactured to be light brightness weight, and has thus been considered one of the next-generation display devices.
FIG. 1 illustrates the construction of a conventional plasma display panel. As shown in FIG. 1, the plasma display panel comprises a front panel 100 and a rear panel 110. In the front panel 100, a plurality of sustain electrode pairs in which a plurality of scan electrodes 102 and sustain electrodes 103 form pairs are arranged on a front glass 101, i.e., a display surface on which images are displayed. In the rear panel 110, a plurality of address electrodes 113 disposed to intersect the plurality of sustain electrode pairs are arranged on a rear glass 111, i.e., a rear surface. The front panel 100 and the rear panel 110 are parallel to each other with a predetermined distance therebetween.
The front panel 100 comprises the pairs of scan electrodes 102 and sustain electrodes 103, which mutually discharge each other and maintain the emission of a cell in one discharge cell. In other words, each of the scan electrode 102 and the sustain electrode 103 has a transparent electrode “a” made of a transparent ITO material and a bus electrode “b” made of a metal material. The scan electrodes 102 and the sustain electrodes 103 are covered with one or more upper dielectric layers 104 for limiting the discharge current and providing insulation among the electrode pairs. A protection layer 105 having magnesium oxide (MgO) deposited thereon is formed on the dielectric layers 104 to facilitate a discharge condition.
In the rear panel 110, barrier ribs 112 of stripe form (or well form), for forming a plurality of discharge spaces, i.e., discharge cells are arranged parallel to one another. A plurality of address electrodes 113, which generate vacuum ultraviolet rays by performing an address discharge, are disposed parallel to the barrier ribs 112. R, G and B phosphors 114 that emit a visible ray for displaying images during an address discharge are coated on a top surface of the rear panel 110. A low dielectric layer 115 for protecting the address electrodes 113 is formed between the address electrodes 113 and the phosphors 114.
A method of implementing images gray levels in the conventional plasma display panel constructed above will be described below with reference to FIG. 2.
FIG. 2 illustrates a method of implementing image gray levels in the conventional plasma display panel.
As shown in FIG. 2, to implement image gray levels in the conventional plasma display panel, one frame is divided into several sub-fields, each sub-field having a different number of emissions. Each sub-field is subdivided into a reset period RPD for initializing the entire cells, an address period APD for selecting a cell to be discharged, and a sustain period SPD for implementing gray levels depending on the number of discharges. For example, to display images with 256 gray levels, a frame period (16.67 ms) corresponding to 1/60 seconds is divided into eight sub-fields SF1 to SF8, as shown in FIG. 2. Each of the eight sub-fields SF1 to SF8 is again divided into a reset period, an address period and a sustain period.
The reset period and the address period of each sub-field are the same. An address discharge for selecting a cell to be discharged is generated due to a voltage difference between the address electrodes and the scan electrodes, i.e., transparent electrodes. The sustain period increases in the ratio of 2n (where, n=0,1,2,3,4,5,6,7) in each sub-field. As described above, since the sustain period is changed in each sub-field, image gray levels are implemented by controlling the sustain period of each sub-field, i.e., a sustain discharge number.
FIG. 3 shows a driving waveform depending on the driving method of the conventional plasma display panel.
As shown in FIG. 3, the plasma display panel is driven with it being divided into a reset period for initializing all of the cells, an address period for selecting cells to be discharged, a sustain period for sustaining the discharge of the selected cells, and an erase period for erasing wall charges within the discharged cells.
In a set-up period of the reset period, a ramp-up pulse (Ramp-up) is applied to all of the scan electrodes at the same time. The ramp-up pulse generates a dark discharge within the discharge cells of the entire screen. The set-up discharge causes positive wall charges to be accumulated on the address electrodes and the sustain electrodes, and negative wall charges to be accumulated on the scan electrodes.
In a set-down period of the reset period, after the ramp-up pulse is applied, a ramp-down pulse (Ramp-down), which starts falling from a positive voltage lower than a peak voltage of the ramp-up pulse up to a predetermined voltage level lower than a ground (GND) level voltage, generates a weak erase discharge within the cells, thereby sufficiently erasing wall charges excessively formed on the scan electrodes. The set-down discharge causes wall charges of the degree in which an address discharge can occur stably to uniformly remain within the cells.
In the address period, while a negative scan pulse is sequentially applied to the scan electrodes, a positive data pulse is applied to the address electrodes in synchronization with the scan pulse. As a wall voltage generated in the reset period is added to a voltage difference between the scan pulse and the data pulse, an address discharge is generated within the discharge cells to which the data pulse is applied. Wall charges of the degree in which a discharge can occur when a sustain voltage (Vs) is applied are formed within the cells selected by an address discharge. The sustain electrode is supplied with a positive voltage (Vz) to reduce between the sustain electrode and the scan electrodes during the set-down period and the address period so that an erroneous discharge is not generated between the sustain electrode and the scan electrodes.
In the sustain period, a sustain pulse (SUS) is alternately applied to the scan electrodes and the sustain electrode. In cells selected by an address discharge, a sustain discharge, i.e., a display discharge is generated between the scan electrodes and the sustain electrodes whenever a sustain pulse is added to the wall voltage within the cell selected by the address discharge.
After the sustain discharge finishes, in the erase period, a voltage of an erase ramp pulse (Ramp-ers) having a narrow pulse width and a low voltage level is applied to the sustain electrodes, thereby erasing wall charges remaining within the cells of the entire screen.
A discharge that may influence the implementation of the gray levels is the address discharge generating in the address period and the sustain discharge generating in the sustain period. Light generated by these discharges is radiated outwardly, thereby implementing gray levels.
FIG. 4 illustrates a discharge affecting the implementation of gray levels in the driving waveform shown in FIG. 3.
As shown in FIG. 4, in a region A of the driving waveform shown in FIG. 3, an address discharge is generated between the scan electrodes Y and the address electrodes X in the address period. In a region B of the driving waveform shown in FIG. 3, a sustain discharge is generated between the scan electrodes Y and the sustain electrode Z in the sustain period. Light generated by the address discharge and the sustain discharge affects the implementation of gray levels. Although a reset discharge is generated in the reset period, the reset discharge is generated within all of the discharge cells on the plasma display panel. Therefore, light generated by this reset discharge does not affect the implementation of gray levels.
In such a conventional driving waveform, an integral multiple of a pair of sustain pulses is applied to the scan electrodes and the sustain electrode in the sustain period of each sub-field. Accordingly, gray levels are implemented upon a display discharge. If the integral multiple of a pair of sustain pulses are applied as described above, the amount of light generated during the sustain period becomes excessive. As a result, a problem arises in that the implementation of the gray levels is deteriorated in low gray level sub-fields for implementing low gray levels.
Another problem arises in that the picture quality is degraded since a substantial amount of half-tone noise is generated by the conventional sustain discharge and the address discharge.
FIG. 5 illustrates an example of a method of implementing low gray levels of 1 or less in the driving waveform shown in FIG. 3.
It is assumed that the light implemented by the driving waveform in the first sub-field SF1 of FIG. 3 is light implementing the gray level 2. As shown in FIG. 5, the number of discharge cells C that are turned off and discharge cells D that are turned on to implement gray levels of 0.5 in a region comprising a total of 16 discharge cells on the plasma display panel is controlled, thus generally implementing gray levels of 0.5. The reason why light implemented by the driving waveform of FIG. 3 is light implementing the gray level 2 is that it is assumed that one sustain pulse implements the gray level 1 for the convenience of this discussion. Since two sustain pulses are supplied in the driving waveform of the first sub-field SF1 of FIG. 3, a total of two gray levels is implemented. Accordingly, one discharge cell that is turned on in FIG. 5 radiates light that implements two gray levels. If a total of three discharge cells is turned off and one discharge cell is turned on in a region 600 comprsising four discharge cells as shown in FIG. 5, the discharge cells of each region 600 consisting of four discharge cells implement a 0.5 gray level. This method employs a person's optical illusion phenomenon, which is one of half-tone schemes.
In the conventional gray-level implementation method, however, a difference in the brightness between the discharge cells that are turned on and the discharge cells that are turned off is relatively high due to an address discharge and a sustain discharge formed as an integral multiple of a pair of sustain pulses is applied. Since the number of discharge cells that are turned on is relatively smaller than the number of discharge cells that are turned off, the picture quality spreads at the boundary portion of images. Accordingly, problems arise in that significant half-tone noise is generated and the picture quality is degraded.